1. Field of the Invention
The present invention relates to an exposure apparatus, an exposure method, a calculation method, and a device fabrication method.
2. Description of the Related Art
A projection exposure apparatus has conventionally been employed to fabricate a micropatterned semiconductor device such as a semiconductor memory or logic circuit by using photolithography. The projection exposure apparatus projects and transfers a circuit pattern formed on a reticle (mask) onto a substrate such as a wafer by a projection optical system.
When the projection exposure apparatus repeats exposure, the projection optical system heats up upon absorbing the energy of the exposure light and then cools down upon dissipating the heat, so fluctuations (to be referred to as “exposure aberrations” hereinafter) in the optical characteristics (e.g., projection magnification and wavefront aberration) of the projection optical system occur. The exposure aberration can be calculated by a computational expression which uses, as parameters, a time constant unique to the projection optical system, an exposure aberration saturation value (exposure aberration correction coefficient) per unit light amount when the projection optical system has reached a thermal equilibrium, the sum total of light which passes through the reticle, and the exposure time. The exposure aberration is corrected by driving a lens which constitutes the projection optical system or controlling the pressure between lenses. Details of these techniques are disclosed in Japanese Patent Laid-Open No. 8-069963.
The exposure aberration correction coefficient changes for each illumination condition, so it must be calculated for each illumination condition. However, calculation of the exposure aberration correction coefficients for all illumination conditions used in the exposure apparatus requires a very large amount of time. For this reason, in practice, exposure aberration correction coefficients are predicted from those for several other illumination conditions. For example, the exposure aberration correction coefficients for several illumination conditions are calculated for each illumination mode, including a normal illumination mode, an annular illumination mode, and a dipole illumination mode. The exposure aberration correction coefficients for illumination conditions, for which calculation is not performed, in the same mode are predicted by interpolating or extrapolating those for the illumination conditions for which their calculation is performed actually.
Unfortunately, in the prior art, illumination modes for which the exposure aberration correction coefficients can be predicted are limited to specific illumination modes, so the exposure aberration correction coefficients must be individually calculated for other illumination modes. This increases the load of calculation of the exposure aberration correction coefficients in proportion to the number of illumination modes used in the exposure apparatus.
In addition, although a light intensity distribution formed on the pupil plane of the projection optical system changes even in the same illumination mode due to the influence of diffracted light from the reticle, the prior art does not take account of the diffracted light from the reticle. This makes it impossible to precisely predict the exposure aberration correction coefficients. It is therefore impossible to precisely correct exposure aberrations generated in the projection optical system, resulting in a decrease in the exposure performances (e.g., the imaging performance) of the exposure apparatus.